Variable gain amplifiers (VGA) are widely used in communication systems, specifically in transceivers incorporating automatic gain control (AGC) loops. AGC loops are used to maintain controlled output signal levels despite changing input signal levels. Often, the gain control unit is implemented by current-steering transistors which are transistors that control the flowing of current throughout the circuit and whose steering ratio is determined by a negative feedback loop of some sort. Current steering transistors may be either bipolar transistors (BJTs) or field effect transistors (FETs) or the combination thereof which is very common in analog and mixed signals applications.
FIG. 1 shows a simplified schematic circuit diagram of a differential amplifier (possibly with variable gain) according to the prior art. The differential amplifier 100 has a negative input (IN_N) 110 and a positive input (IN_P) 120, a negative output (OUT_N) 130 and a positive output (OUT_P) 140. These signals may be voltages, currents, or powers; depending on the circuit. Additionally, the differential amplifier 100 has a gain control pin 150, whereby the gain of the differential amplifier 100 is proportional to a control signal on this pin. This control signal is generally a DC (zero frequency) signal to enable control simplicity, as opposed to the input and output signals, whose frequency is dependent on the application. The transfer function of differential amplifier 100 gain to control signal 150 amplitude, where amplifier gain is equal to the output signal level (i.e., the amplitude of the difference between the signals Out_N and Out_P) divided by the input signal level (In_N minus In_P), is generally nonlinear. A simple open loop control mechanism may be implemented with the transfer function 170, wherein the control signal 180 may be scaled by some gain or attenuation. Alternately, the transfer function 170 may be nonlinear, to implement some favorable relationship between the differential amplifier gain and the control signal 180, such as linear-in-dB. This differential amplifier may optionally employ a feedback loop in order to desensitize the output signal level to external perturbation and variations over process, temperature, and voltage, being comprised of the signal detector 160 and the transfer function 170, in conjunction with the differential amplifier 100. A signal detector 160 produces a zero frequency (DC) signal which is proportional to the amplitude of the signal at 130, 140. In a minimal configuration, the transfer function 170 may simply be a subtraction, realizing a simple negative feedback loop intended to make the output of the signal detector 160, and thus the signal levels at 130, 140 constant and equal to the control signal 180. More elaborately, the transfer function 170 may include some gain or attenuation to scale the control signal 180, or may be a nonlinear transfer function in order to implement some favorable relationship between the control signal 180 and the output signal levels at 130, 140, such as a linear-in-dB relationship.
Variations in semiconductor process, temperature, and/or voltage will, in general, produce variations in the slope of the transfer function of differential amplifier 100 gain to the amplitude of the control signal 180; henceforth the slope of this transfer function will be called control gain. In the open loop case, control gain variation is minimized by having the transfer function 170 vary over PVT in an inverse manner as differential amplifier 100. In the closed loop case, the transfer function of the signal detector 160 needs to vary minimally over PVT. Either of these are difficult to assure, in general.
A VGA with current steering feedback is described in D. Su and W. McFarland, “An IC for Linearizing RF Power Amplifiers Using Envelope Elimination and Restoration.” IEEE J. Solid-State Circuits, vol. 33, pp. 2252-2258, December 1998. (“Su and McFarland”). This VGA is a radio frequency (RF) design that uses a multiplier-like structure with two differential amplifiers to ensure a constant DC bias point at the drains of the differential amplifiers. The VGA is operated in conjunction with a limiter which utilizes it for feeding into an envelope detector, while a control amplifier tries to keep the output of the envelope detector the same as a reference voltage. Thus, the VGA according to Su and McFarland implements a feedback in order to linearize its response. It has a constant output common mode level, but does not implement the gain control using common mode feedback. Additionally, this VGA provides poor performance in most RF aspects. It also cannot retain a linearized control if removed from the limiter and so it cannot be used standalone. Other methods of linearization, which employ control linearizing circuits, exist but are prone to variation and mismatch over process, voltage, and temperature.
A fundamental drawback of presently available designs is their tendency to vary in their performance significantly as a result of changes made in the integrated circuit (IC) fabrication process (W/L ratio in FET transistors, base area in BJT transistors etc.), voltage and temperature variations (hereinafter: PVT).
Takafumi Yamaji and Nobuo Kanou offer a partial solution for temperature variations in their article “A Temperature-Stable CMOS Variable Gain Amplifier With 80-dB Linearly Controlled Gain Range” J. Solid-State Circuits, vol. 37, No. 5 pp. 553-558, May 2002. (“Yamaji and Kanou”).
Migrating from one process corner to another or operating in a different temperature/voltage region than the original design requirements may result in severe underperforming. Therefore, a controllable VGA that is robust enough to overcome process, voltage and temperature (PVT) variations may reduce development costs and shorten the time-to-market of new designs.